References voltages used in circuits such as memories should be stable and immune from temperature and power supply variations since overall circuit performance is negatively effected by any variation in the reference voltage. Therefore, a bandgap reference voltage generator is often employed to generate an internal voltage reference for integrated circuits.
The operating principle behind bandgap reference voltage generation is familiar to those in the art but is briefly described below. A bandgap reference voltage generator is a voltage reference circuit widely used in integrated circuits, usually with an output voltage around 1.25 V, close to the theoretical bandgap of silicon at 0 K. The voltage difference between two unequal size diodes is used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current is used to generate a PTAT voltage in a second resistor, which is added to the voltage of a diode, in some implementations. If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the diode voltage and the PTAT voltage will be canceled out. The resulting voltage is around 1.25V. The voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts.
Examples of prior art bandgap circuits are provided in U.S. Pat. No. 6,788,131 to Huang and U.S. Pat. No. 5,200,273 to Mao, the entirety of which are hereby incorporated by reference herein.
In summary, the output voltage is made substantially invariant with regard to temperature by taking a weighted sum of a voltage that has a negative temperature coefficient (viz the voltage across the PN junction) and one that has a positive temperature coefficient. However, the bandgap reference voltage generator is always “on” or “enabled” in order to provide the reference voltage for the integrated circuit, thereby increasing the power consumption of the integrated circuit. Lower power reference voltage generation circuits and methods, therefore, are desired.